The present invention relates generally to wireless broadcast transmissions and more particularly to a method for optimizing the delivery of IP multicast group packets, especially in an 802.11 network.
Unless otherwise defined herein, the terms in this specification should be interpreted as defined, or as customarily used, in the Institute of Electrical and Electronics Engineers (IEEE) 802.11 and 802.11e specifications. The IEEE 802.11 and IEEE 802.11e specifications are hereby incorporated by reference. The current draft standard refers to the current draft supplement to the 802.11e specification, which is hereby incorporated by reference.
An 802.11 Local Area Network (LAN) is based on an architecture wherein the system is divided into a basic unit or cell which is known as the Basic Service Set or BSS. The BSS is controlled by a base station commonly designated as an Access Point (“AP”). In an 802.11 BSS, if a single station enters Power-Save Protocol (PSP) operation, the entire BSS adopts different characteristics to be able to provide services to that one station. Under normal use, this would not present too great a hardship as slight lag and delay times for transmission would not be noticeable for data transfers. However, for transmissions of more urgency, such as multicast, Voice-over-Internet Protocol (VoIP), video, etc., such a lag and delay time becomes noticeable to the user. In a BSS where one of the stations has gone into PSP operation, the BSS-wide characteristics will have changed to provide the PSP station with service. One change made is that all multicasts and broadcasts are queued by the AP to be transmitted as a batch after the AP transmits its next 802.11 beacon containing a Delivery Traffic Indication Map (DTIM). For example, station A and station B are both clients of an 802.11 AP. When station A goes into 802.11 PSP operation, station B remains active. If station B then subscribes to an IP multicast group, the 802.11 AP will buffer the IP multicast stream to compensate for station A being in PSP operation, even though station A is not a subscriber to the IP multicast group.
Internet Group Multicast Protocol (IGMP) was developed to allow stations to request membership into a multicast group or video stream/conference. IGMP is not used by the application for actual voice/video transfer, but rather as a membership mechanism for joining or participating in multicasts. IGMP is itself a session-layer protocol used to establish membership in a multicast group. The method of transport for multicast data will traditionally be RTP. Membership into a multicast group is dynamic, and members can enter and leave the group at any time. A user (host) can be a member of one or more groups simultaneously, and does not need to be a member of a group in order to send datagrams to it. A “host” can be a user or a multicast router.
In accordance with the forgoing, there is a need to allow for the immediate transmission of IP multicast data streams for which there are only active 802.11 subscribers.
There also is a need to make the quality of IP multicast data streams no longer dependent on all stations in a BSS remaining active.
Thus there is a need to offset the delays in transmission as a result of a station entering power-save operation in a BSS for other stations subscribing to IP multicasts.